1. Field of the Invention
The invention relates to a device for operating a high pressure discharge lamp which is used for a liquid crystal projector and the like.
2. Description of Related Art
Ordinarily, a circuit as shown, for example, in FIG. 5, is known as a lighting circuit for operating a high pressure discharge lamp with rectangular waves using an electronic circuit. In the circuit shown in FIG. 5, a lighting circuit is formed from a full bridge circuit 2 which is formed of a switching device Q1, a DC source, switching devices Q2 to Q5, diodes D2 to D5, and an ignition coil 4.
The lighting circuit shown in FIG. 5 alternately turns on the switching devices Q2, Q5 and the switching devices Q3, Q4 of the full bridge circuit 2 by supplying a voltage and a current from the direct current source to the full bridge system 2. Thus, a high pressure discharge lamp 3 is supplied with an AC voltage with rectangular waves and is operated in this way. When the high pressure discharge lamp is started, the ignition coil 4 applies a high voltage pulse to the lamp 3, by which the lamp is started.
In the device for operating a lamp with the above described arrangement, normally, a time is taken which is called the dead time Td and in which the switching devices Q2 to Q5 of the full bridge circuit 2 are all switched off in order to prevent a cross current of the full bridge circuit 2 and to prevent damage of the switching devices Q2 to Q5 and the switching device Q1 for controlling the wattage 2.
There is also a case in which, without the dead time Td, with a reactance coil L1 which is located on the rear stage of the switching device Q1 for controlling the wattage, the cross current of the bridge circuit is limited to less than or equal to a certain value and by which damage to the components is prevented. However, here, the disadvantages of an increase in the damage to components, formation of noise and similar disadvantages often arise. Therefore, normally, the dead time is chosen in general.
However, if rectangular waves with a low frequency with a dead time are applied to the high pressure discharge lamp, there are cases in which there are the disadvantages that the current which flows in the high pressure discharge lamp is interrupted, the lamp goes out and a re-ignition spike or the like occurs.
To prevent current interruption by the dead time Td, for example, the measure which is described in Japanese patent disclosure document HEI 6-101388 (U.S. Pat. No. 4,734,624) is known. In this publication, an inductance is connected in series to the lamp, there is a capacitor parallel to this series connection, and thus, a LC-resonant circuit is formed. During the dead time, a series resonance is formed with the rectangular waves by the above described inductance and the above described capacitor, and an oscillating current is allowed to flow in the lamp. For this reason, the attempt is made to eliminate the time during which current does not flow.
However, in the process described in the aforementioned publication, a large capacitor, for example, in which C3=0.1 xcexcF, is connected on the rear stage of a full bridge circuit. Therefore, it is difficult to obtain a steep output of the rectangular waves of the switching devices Q2 to Q5 of the full bridge circuit 2, resulting in the undesirable phenomenon that the radiant light from the high pressure discharge lamp instantaneously darkens for a moment. This reduction of the brightness in an application as a light source of a projector device of the projection type is a serious disadvantage. Furthermore, a large L2 of 1 mH must be used, by which a reduction in the size and weight of the luminous current source is difficult.
When C3 and L2 have the above described values, the oscillating current which flows in the high pressure discharge lamp has a frequency of 16 kHz and a period of roughly 63 xcexcs. In this way, a change of the waveform, overshoot and undershoot occur more often.
The invention was devised to eliminate the above described disadvantages in the prior art. A primary object of the present invention is to prevent lamp extinction during the dead time in a device for operating a high pressure discharge lamp using an inverter circuit of a full bridge system in which a dead time is taken, and in which, furthermore, switching devices are driven to prevent lamp extinction during the dead time, to reduce the delays of the rising time and the falling time of the rectangular waves which have been output by the inverter circuit by the effects of the inductance L, of the capacitor C and the like, and to prevent formation of the phenomenon of instantaneous darkening of the radiant light.
The object is achieved as in accordance with the invention as follows:
(1) In a device for operating a high pressure discharge lamp with a feed device with an inverter circuit in which four switching devices are connected in a bridge-like manner, in which a dead time is taken in which all switching devices are turned off, and which thus feeds the high pressure discharge lamp with an alternating current, on the forward stage of the inverter circuit there is a capacitor for forming a loop which in the above described dead time supplies a current to the high pressure discharge lamp and moreover on the rear stage of the inverter circuit there is an inductance element which is connected to the high pressure discharge lamp in series and which together with the above described capacitor forms the above described loop. The value LL of the inductance L of the above described inductance element is fixed at a value that is greater than or equal to the value at which a current can be supplied without interruption to the high pressure discharge lamp during the above described dead time by the energy stored in this inductance. This means that the value LL of the above described inductance L is established such that LLxe2x89xa7VL/ILxc2x7Td where VL is the luminous voltage of the discharge lamp, IL is the current flowing in the discharge lamp and Td is the dead time in Td (seconds).
(2) In (1), at least part of the inductance element is formed from an ignition coil transformer.
By the above described arrangement in accordance with the invention, even during the dead time, in the high pressure discharge lamp, a current can be allowed to flow by the energy which has been stored in the above described inductance element, and thus, lamp extinction which is produced by the dead time of the alternating rectangular waves can be prevented. Furthermore, the rising and falling of the alternating rectangular waves can be made steep because the value LL of the above described inductance element need not be large. Thus, the disadvantage of an instantaneous darkening of the high pressure discharge lamp can be eliminated and furthermore a reduction in the size and weight of the lighting current source can be achieved.
The invention is described below using several embodiments shown in the drawings.
FIG. 1 shows a schematic of the first embodiment of the arrangement of a device in accordance with the invention for operating a high pressure discharge lamp;
FIG. 2 is a plot of the current which flows during the dead time in a discharge lamp;
FIG. 3 is a schematic of the second embodiment of the arrangement of a device in accordance with the invention for operating a high pressure discharge lamp;
FIG. 4 is a schematic of the third embodiment of the arrangement of a device in accordance with the invention for operating a high pressure discharge lamp; and
FIG. 5 shows a schematic of one example of the arrangement of a known device for operating a discharge lamp.